As disclosed in Japanese Patent Laid-Open No. 2006-120679, 1/f noise is reduced when the gate of an amplifier transistor included in each pixel of a MOS image sensor is formed to have a buried channel structure. As disclosed in the paragraph “0034” of Japanese Patent Laid-Open No. 2006-120679, as a method of forming the buried channel structure, when the amplifier transistor is a MOS transistor of a first conductivity type, its gate electrode is formed by polysilicon of a second conductivity type. As disclosed in the paragraph “0062” of Japanese Patent Laid-Open No. 2006-120679, a PMOS transistor can be used as the amplifier transistor in place of an NMOS transistor.
Japanese Patent Laid-Open No. 2006-120679 has not examined about channel structures of transistors in a peripheral region as a region other than a pixel region. If that reference pursuits process simplification, all MOS transistors of an identical conductivity type (for example, PMOS transistors) in a pixel region and peripheral region should have an identical channel structure. In this case, all MOS transistors of the same conductivity type as that of the amplifier transistor in the pixel region may have a buried channel structure.
However, a MOS transistor having a buried channel structure (for example, a PMOS transistor having an n-type gate) tends to be a normally ON transistor. For this reason, a leak current may increase, and consumption power may become high.
Also, Japanese Patent Laid-Open No. 2006-120679 has not examined about a structure of a channel stopper region in a MOS transistor. Since the amplifier transistor of each pixel is arranged adjacent to a photodiode, it should be isolated by an element isolation structure including a channel stop region so as to suppress the influence of a dark current. On the other hand, since a column selecting circuit requires a miniaturized transistor structure more than transistors which form each pixel, it is important to reduce an ON resistance of the transistor, but necessity of consideration of a dark current is low. The channel stop region arranged in the column selecting circuit not only increases an ON resistance but also disturbs miniaturization, resulting in disadvantages rather than advantages. When a portion that especially requires a high-seed operation in a peripheral circuit (in particular, a MOS transistor of the column selecting circuit (also called a column decoder or horizontal scanning circuit)) uses the same structure as the amplifier MOS transistor of each pixel, an image sensing device having a sufficiently high readout speed cannot be obtained.